New structural data reveal benleonardite to be a member of the pearceite-polybasite group
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The determination of the crystal structure of benleonardite (S.G. P3m1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag15.0Cu1.0Sb1.58As0.42S7.03Te3.97) obtained using data from a gem-quality, untwinned crystal recovered from the type material, revealed that benleonardite exhibits the structure observed for minerals of the pearceite-polybasite group. The structure consists in the stacking of [Ag6(Sb,As)2S6Te]2- A and [Ag9Cu(S,Te)2Te2]2+ B layer modules in which (Sb,As) forms isolated SbS3 pyramids 20 typically occurring in sulfosalts, Cu links two (S,Te) atoms in a linear coordination, and Ag occupies sites with coordination ranging from quasi linear to almost triangular. The silver d10 ions are found in the B layer module along two-dimensional diffusion paths and their electron densities, which are evidenced by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split model. In the structure, two S-positions are completely replaced by Te (i.e., Te3 and Te4) and one for one half [S1: S0.514(9)Te0.486], whereas S2 is completely filled by sulfur. This distribution reflects on the crystal-chemical environments of the different cations. On the basis of information gained from this characterization, the crystal-chemical formula of benleonardite was revised according to the structural results, yielding Ag15Cu(Sb,As)2S7Te4 (Z = 1) instead of Ag8(Sb,As)Te2S3 (Z = 2) as previously reported. Thus, the mineral must be considered a member of the pearceite polybasite group. A recalculation of the chemical data listed in the scientific literature for benleonardite according to the structural results obtained here leads to an excellent agreement.